The present invention relates to a gas generator for actuating vehicle occupant restraint devices such as pretensioner belts.
A pretensioner belt is a vehicle occupant restraint device for detecting collision and subsequently tightening a seat belt. For tightening seat belts, there has been normally employed a mechanism of tightening seat belts by gas pressure of a gas generator.
Such gas generator includes a first hollow body with a bottom and sides, gas generants filled in the first hollow body, an electric ignitor, and a holder. The electric ignitor is formed with a second hollow body with a bottom and sides, igniting agents housed in the second hollow body, and a plug closing an opening portion of the second hollow body. The holder positions the second hollow body in the center of the first hollow body, holding the plug of the electrical ignitor. The holder fixes the second hollow body in the first hollow body, closing the opening portion of the first hollow body.
In this kind of as generator, two parts are prepared. One is a first hollow body filled with gas generants. The other is an electric ignitor fixed to a holder and shaped so as to project from the holder. It is difficult to put the holder into the first hollow body without leaving an excessive empty space because of the cylindrical configuration of the was generator.
Therefore, when the electric ignitor is actuated and the gas generants are ignited, increase in inner pressure by ignition in the first hollow body is slowed because of the excessive empty space in the first hollow body. As a result, a rupture of a rupture member of the first hollow body is delayed and there is an adverse possibility that desired performance cannot be obtained.
Generally, a permissible ignition delay period (the period of time between a start of current flow to the electric ignitor and a start of discharging gas from the gas generator) is within 2 ms for the seat belt pretensioners of automobiles. When the ignition delay period is more than 2 ms, an occupant restraint performance cannot be shown sufficiently.
when excessive empty space exists, a size of a gas generator is extended by the empty space. Further, because the gas generants are not fixed, powdering thereof by vibration of a vehicle is also concerned. When such a powdering phenomenon occurs, a combustion velocity of the gas generants becomes extremely fast, and then the gas generator may possibly be damaged.
Hence, Japanese Patent Application Laid-Open No. Hei 8-207694 has disclosed a cup-shaped filling cylinder for filling gas generants without leaving an empty space. The second hollow body of the electric ignitor projects inwardly from the bottom of the cup-shaped filling cylinder. The cup-shaped filling cylinder opens upwardly from the second hollow body of the electric ignitor.
First, in the gas generators having the above-mentioned cup-shaped filling cylinder, the gas generants are filled in the cup-shaped filling cylinder thereof without leaving an empty space. Then, the first hollow body is put on the cup-shaped filling cylinder to cover the opening portion of the cup-shaped filling cylinder. When the gas generants are filled in the first hollow body in this way, it is possible to fix the second hollow body of the electric ignitor in the first hollow body without forming an excessive empty space therein.
However, a filling cylinder needs to be newly provided in this case. As a result, in addition to an increase in the number of components, simplification of the manufacturing process cannot be expected.
It is an object of the present invention to provide a gas generator for actuating vehicle occupant restraint devices having a small number of components and excellent ignitability, and being capable of miniaturization.
As a result of eager investigation of the present inventors to solve the above-mentioned problem, the present inventors have found that it is important to regulate a ratio of an empty space to an inner volume of the first hollow body to achieve the present invention.
The present invention relates to a gas generator for actuating a vehicle occupant restraint device comprising a first hollow body with an end and a side wall, gas generants densely filled in the first hollow-body, an electric ignitor formed by housing igniting agents in a second hollow body with an end and a side wall then closing the second hollow body with a plug, and a holder positioning the second hollow body in the center of the first hollow body while fixing the first hollow body and holding the plug of the electric ignitor.
In a first gas generator of the present invention, a ratio of an empty space to a volume of a space partitioned by an inner surface of the first hollow body, an outer surface of the second hollow body and the holder (hereinafter it is described as full volume) is less than 20% by volume. The empty space and the ratio of the empty space by volume are calculated by using
empty space=full volumexe2x88x92filling volumexe2x80x83xe2x80x83(1)
ratio of an empty space by volume=(empty space volume/full volume)xc3x97100xe2x80x83xe2x80x83(2)
Fundamentally, a space that exists among the particles of filled gas generant compositions is not regarded as a gap. And the filling volume means a necessary volume for gas generants filled in a dense state to substantially exist in the full volume.
According to the first gas generator of the present invention, because the ratio of the empty space to the full volume of the first hollow body is less than 20%, the first hollow body is substantially full of the gas generants. Therefore, ignition energy of the electric ignitor is transmitted efficiently and the time for raising for inner pressure of the first hollow body is shortened.
Accordingly, in the case an electrical ignitor with small ignition power is used, there is no possibility that an ignition delay occurs. In addition, because the first hollow body is substantially full of the gas generants, the gas generants are prevented from being powdered by vibration of vehicles. To ensure the above-mentioned action, it is preferable that the ratio of empty space is less than 15%, further preferably, less than 10%.
The conventional cup-shaped filling cylinder is not necessary, thereby providing reduction of the number of components.
A second gas generator of the present invention is characterized in that the gas generants are powdery or granulated and filled in a compressed state, in addition to the above-mentioned characteristics of the first gas generator. As examples of compressible gas generants, there are powdery or granulated non-azide gas generants, propellant gas generants and the like.
According to the second gas generator of the present invention, the gas generants are powdery or granulated. Therefore, the gas generants can be filled without leaving space and a ratio of the empty space can be easily predetermined. Because the gas generants are filled in a compressed state, the filling density is heightened and the gas generator is miniaturized. Combustion velocity can be adjusted by adjusting a filling weight and a compression height (i.e. a height of a surface formed with the filled agents). It is preferable that the average particle size of material of the powdery or granulated gas generants is adjusted so as to fall into the range of 10-300 xcexcm including 10 xcexcm and 300 xcexcm. It is preferable that the compression degree of the gas generants falls into the range of 30-100% including 30% and 100%. The compression degree. of the gas generants is regarded as 100% when the gas generants are compressed with the density equal to the true density.
The powdery or granulated gas generants, adjusted to be in the predetermined range mentioned above, can be hardened into a predetermined form by compression at a predetermined pressure.
A third gas generator of the present invention is characterized in that a part of the full volume is filled with a spacer 7 inserted between an outer surface of the side wall of the second hollow body 4a and an inner surface of the side wall of the first hollow body 2, in addition to the above-mentioned characteristics of the first or the second gas generator.
According to the third gas generator of the present invention, an empty space between the outer surface of the side wall of the second hollow body 4a and the inner surface of the side wall of the first hollow body 2 can be filled with the spacer 7. Therefore, even if a form of the gas generator cannot be modified for reason of a design of a vehicle occupant restraint device, it is possible to lower the ratio of the empty space. As a result, the first hollow body 2 is substantially full of the gas generants 6, thereby an excellent ignitability is obtained.
An empty space in this gas generator is calculated by using
xe2x80x83empty space=full volumexe2x88x92(filling volume+volume of a spacer)xe2x80x83xe2x80x83(1xe2x80x2)
Further, when the length of the spacer is adjusted according to the lengths of the first hollow body and the second hollow body, it is possible to adjust the full length of the gas generator without eliminating an excellent ignitability.
Particularly, a configuration where the spacer is located around the electric ignitor, is desirable. The electric ignitor may be formed so as to include the spacer depending on circumstances. Thereby, ignition energy is concentrated on the gas generants, and the ignitability is maintained. The spacer remains fixed on the electric ignitor after the gas generator is actuated.
Rigid material is desirable for the spacer. For example, the spacer is made of aluminum, plastic, glass fiber, and the like and formed into a doughnut shape, a mesh shape, and the like. When the spacer is made of a gas generant composition with the same form as mentioned above, it is possible to increase the output of the gas generator.
A fourth gas generator of the present invention is characterized in that the gas generants are compressed in advance and filled in the first hollow body so that a concavity, in at which the second hollow body is inserted, is formed, in addition to the above-mentioned characteristics of the first or the second gas generator.
According to the fourth gas generator of the present invention, the gas generants are compressed in the first hollow body in advance so that the concavity, in which the second hollow body is inserted, is formed. Therefore, when the second hollow body is inserted into the first hollow body, the second hollow body is surrounded by the gas generants. The ignition energy of the electric ignitor is transmitted to the gas generants surrounding the second hollow body without waste. Therefore, there is no possibility that an ignition delay occurs. For compressing the gas generants into a concave shape in the first hollow body, convex compressing tools may be used.
A fifth generator of the present invention is characterized in that gas generants that are incompressible or hard to compress are filled densely in the first hollow body and a part of the full volume is filled with the spacer inserted between the outer surface of the side wall of the second hollow body and the inner surfaces surface of the side wall of the first hollow body, in addition to the above-mentioned characteristics of the first gas generator.
In the gas generator of the present invention described above, the excessive empty space in the first hollow body can be reduced, thereby reducing an ignition delay of the gas generator. Further, the gas generants are prevented from being powdered by vibration of vehicles. Thus, the gas generator of the present invention is excellent in ignitability and reduced in size though it is manufactured in a manufacturing process almost the same with the conventional one.